FIG. 3 shows a known direction indicating apparatus for an automobile or a motorcycle. The direction indicating apparatus uses bulbs as direction indicating lamps (blinkers) 102L and 102R.
For example, when a direction indicating switch “SW1” is switched to an L terminal side, an oscillator 104a generates an oscillation pulse signal. A buffer 104b buffers the oscillation pulse signal and outputs a pulse signal. An N-type MOS transistor “NM101” flows a drive current having a magnitude of the amplitude of the pulse signal supplied to the gate thereof from the drain to which a power supply voltage “VDD” to the source. The drive current flows to bulbs 102LF and 102LR of the direction indicating lamp 102L via a current detecting resistor “R101” and the direction indicating switch “SW1”. In this way, a direction indicating operation occurs in which the direction indicating lamp 102L blinks at the frequency of the oscillation pulse signal. If a hazard switch “SW2” is turned on, the drive current flows to the direction indicating lamps 102L and 102R via the hazard switch “SW2”, and a hazard operation occurs in which all the direction indicating lamps 102L and 102R blink. In the hazard operation, the drive current is higher than in the direction indicating operation, and the current flowing to the direction indicating lamps 102L and 102R does not significantly change compared with that in the direction indicating operation.
It is known that the power supply voltage from a battery “B” used in a motorcycle or the like significantly changes with the status of the motorcycle body. For example, in a 12V system, the power supply voltage changes approximately from 8V to 15V. In addition, the bulb has a characteristic that the temperature of the filament in the bulb increases as the voltage increases, and the current flowing through the bulb decreases as the resistance of the filament increases. Therefore, even if the power supply voltage increases, the current flowing through the bulb does not significantly increases, and therefore, there is no practical problem.
In JP1-90831A, another known direction indicating apparatus similar to the direction indicating apparatus described above is described.
In recent years, there has been a demand that an LED element be used instead of the bulb as the direction indicating lamp of the direction indicating apparatus in order to improve the styling of the automobile or reduce the power consumption of the electronic equipment. The LED element has a characteristic that the current increases as the voltage increases. As described above, the power supply voltage can significantly change. If the current is regulated with respect to low power supply voltages, a high power supply voltage can lead to a damage to the LED element because the current increases to cause the LED element to generate heat or because the current exceeds the rated current. If the current is regulated with respect to high power supply voltages, a low power supply voltage can lead to a darkening or turning off of the LED element because the current decreases. In order to elongate the service life of the LED element and keep a constant brightness, it is required to provide a constant current circuit that keeps the current flowing through the LED element constant even if the power supply voltage changes.
However, in the hazard operation, the number of direction indicating lamps serving as loads increases twice to four compared with the number of direction indicating lamps in the direction indicating operation. Therefore, if the constant current circuit is provided, there is a problem that the current flowing through the LED element in each direction indicating lamp decreases to approximately half of the current in the direction indicating operation, and the LED element darkens.